As someone who has used the solution you're proposing extensively (much the horror of the data purists out there, I'm certain): You can do that - there's no mechanism in Beeminder to stop you - but I recommend you don't. If you're anything like me, you'll never "catch up". That's why Beeminder exists in the first place.
Once you've taught yourself that dummy data is a way out of a tight spot, it becomes a hard habit to break. I did this a lot when I first was using Beeminder. After a while, I realized that a) I felt guilty about it all the time, b) it meant I hadn't successfully outsourced the tracking effort, because I had to manually track how much I needed to "catch up", and most importantly c) I wasn't achieving the things I wanted to. So I weaned myself off the habit using Beeminder.
As you can see, it took a year to fully break the habit. My recommendation is you don't get started on it in the first place.
It's hard to know what the right rate is when you're first starting out. That's why you have the option to set a $0 initial commitment for a goal - it lets you see what sort of rate you can succeed (or not) at. Most of my goals either derail at least once or have to be retroratcheted because I made them too easy.
In general, I find that an "emergency" (about-to-derail) day is a good time to think about how I got in that situation. There are several possible situations:
Situation: I messed something up while setting the rate, and it's not what I meant it to be.
Action: I email support and they fix it for me.
Situation: I made the goal too hard in the first place, and the rate is one I can't really sustain. I didn't expect this because I've never tried to apply consistent daily effort to this topic before.
Action: I let the goal derail at the $0 level and adjust the rate to a more sustainable level. (If the goal is brand new, deleting it or emailing support for help are options I might pursue as well.)
Situation: Something unexpected happened in my life that I couldn't have reasonably anticipated when I made the goal (illness, family member needing help, technology problems, etc).
Action: I email support and explain the situation. They are very responsive, willing to believe whatever I tell them (unless I've checked "weaselproof me"), and generally will do what they can to make the situation right.
Situation: This is a "goes on forever" goal that I've been doing for a while and consistently skate the edge of. I just need a break, but didn't realize this soon enough to schedule one and/or build up some safe days.
Action: I pay for a derailment and get a break. I generally keep the pledge cap on these sorts of goals low enough that I'm willing to pay $X for a few day's break, but not so low that there's no sting at all. For me that means in the $5 to $30 range, but I suppose it depends on one's financial situation.
Situation: I've been slacking, but I can do at least one day's worth of action today. (After all, one day's worth of the original rate is all I need to do to avoid derailing today).
Action: I do one day's worth of action, and the situation repeats tomorrow. If I do this every day, I will successfully reach my endpoint, but every day will be an emergency day. (Most of my goals are like this for long periods at a time. It's risky, because I eventually end up in the above situation, but it's a good illustration of the reason Beeminder exists.)
Situation: I have been slacking, and I can't summon up the motivation today to do even one day's worth of activity.
Action: I let the goal derail, and hope that the next pledge level will be a little more motivating.
If your case is one of the first two, I'd recommend emailing support and seeing whether they can help you find the right solution for your particular situation. They're generally even more flexible with newbees like yourself than they are with folks like me.