With that set up, you can hit record, and work on the drums on another channel while the mp3 plays back, and record the drums that way. Bearing in mind I just have the fruity edition, so I can't actually record audio directly into FL Studio and I don't feel like upgrading it really (which is why i'm considering switching over to reaper since I've got a couple hardware synths now) And you can drag the playlist bar around to time when sample begins playing back. It's a bit of janky work-around, and there's probably a better way to do it, but it's what I used to do with audio I imported from recording in audacity. Also you'll need to look at the piano-roll for the channel (looks like a little keyboard icon) and make sure a note is active so it will play the audio for the length of the sample. The bar will need to be dragged/lengthened to be the same length as the sample else it will loop on itself. One thing you can do, is drag the mp3 file into a channel in FL Studio, which will treat it as an audio sample, and in the playlist on the right, create a bar for it.

Fl studio only records one bar