If I understand you correctly, and you mean that the Play button on your Nektar is already mapped to the Play button in Logic's control bar, then you should be able to press that Play button during recording to stop recording without stopping playback.
But indeed, Jakob's solution works if you'd rather use the Record button to perform the same action.
reading back my post I could have certainly made that more clear, sorry about that.
I mapped the play button of my Nektar to the play button of Logic using the Learn New Assignment suggestion. After that, when I was pressing the Nektar's play button while recording the recording button was disabled but the playhead would jump back to the beginning of the loop instead of just continuing playback. It would jump back and continue playback from there.