If your sanitation is good, then you should be just fine. I have often used my own bottle-conditioned beer as a bank for yeast strains with limited availability. Just be sure to pay exceptional attention to your sanitation, and step up the starter slowly over a few steps and you'll be fine.
I start by adding about 15 ml of starter wort to a equal amount of beer/dregs in the bottle. That will give you a small starter that has been diluted to half-strength, and still has some alcohol and a lower pH to protect it while the yeast get going slowly. Then I pitch that (after about 5 days) to about 150 ml of normal starter wort, and then step it up once again if needed.