As other posters have said:
1. Yes, you can take the USMLE in addition to the COMLEX
2. Many DO schools require certain steps of COMLEX to graduate
however, in addition:
3. In order to practice you have to take the 3 steps of the COMLEX. You cannot practice as a DO without COMLEX (so in answer to the original poster, no you can't take USMLE "instead", just in "addition" if you feel like it).
4. Melancholy: Usually all that is needed is step 1 of USMLE as the match, which is where these numbers are used, is usually before step 2. However, some people delay match interviews/do step 2 early so that they can present those scores to particularly competitive residencies IF they think they are going to shine on step 2. Most people however do not go this far. There is certainly no point taking USMLE step 3 as that is normally taken during internship.
5. Taking these additional exams will also cost money