Step 1 To switch from async to sync, you can just throw one higher throw (4x instead of 3). To switch back, you simply start the cascade when the single ball is coming down.
The siteswap notation for mixed sync and async patterns is a bit complicated. There are new throws available: 1x is a short hold, 3x which is a low throw to the same hand, and 5x is a high throw to the same hand. Exclamation mark denotes that there is no brake after the synchronous throw, and I use it only when a ball is held in the hand. Juggling Lab also sometimes requires you to use R or L to indicate which hand goes next when switching from sync to async.
Var 1 There are many ways to switch, here is a variant.
Var 2 Here is my favourite variant. By throwing 4x and 3, the change of rhytm happens when the balls are landing rather than when they are thrown.