You are right. On the OT you can only copy to a pattern when you switch to it. You need to think a bit different when it comes down to transitions …
To build while you perform you can use the crossfader transition trick. All you need to prepare beforehand is the setup of a record/playback loop track (T1 for example) and some scene settings on the other tracks on all patterns you want to use (just prepare a single pattern and copy it then around beforehand).
Check the following extensive thread to see how the crossfader transition trick works in detail …