You can use Tenacity, but that's about it. Anything else has to wait until whatever the package recommends for new seed--for the most part, that's usually about the second mowing, but it does vary widely.

Even Tenacity can be a little touchy in this stage--it's generally used at seeding, or one month later. If it's possible to wait just a few weeks, that would be best. At this point, the crabgrass has already done the damage it's going to do for the most part by seeding out, so waiting a couple more weeks really won't hurt.

Don't do it, crabgrass will die out on its own shortly.