Here is one of the fangraphs articles I was talking about earlier saying the break even point is 66%.
The break-even rate on steals has fallen from 68 percent to 66 percent, down from 70 percent at the height of the steroids era in 2000.
It always varies and depends on the calculation and the timespan used. You also have to take into account outs in the inning and base being stolen if you want to really break it down. Yet 70% is a good rule of thumb. So as long as Hamilton is above that break even point, stealing bases is a benefit. No real need to do some complex analysis on his OBP and slugging. It has been done for you.
edit: So, to simply answer your question, you can actually up his OBP when comparing it to other hitters as long as he is stealing bases above a 70% clip. It needs to be broken down into runs and that has all been done elsewhere. If you want to convert it back to OBP to compare him to a person that doesn't steal bases and only compare OBP, figure out what his steals are worth and what getting on base is worth in terms of runs. It's all out there. I don't feel like going through it all.