It’s easy – begin with a 1-bar pattern, and as soon as you’re ready, just extend it to 2 or more bars. Any data from the first bar in all tracks automatically gets copied to subsequent bars. (i.e., jeffe’s right, but you don’t even need to copy it to another pattern first.)
You can also do this with a 2-bar pattern then extend it to 4 bars.
Another cool thing jeffe alludes to is when you shorten a long pattern, you can still lengthen it back again later and the data in the bars you lopped off will still be there. Great for jamming. 