If it's an instrumental part you need to isolate a good version of the phrase you were trying to record, then cut and paste it to fill the gaps.
If it's a vocal part, consider singing it again.
Either way, there's no command on the BR-600 that will do what you're looking for --- slide existing data forward or backward in time to fill time in the tracks. You can do this yourself by using the 'track erase' command to trim the length to after you've cut + pasted a well played section of the material in question. The BR-600 lets you specify the number of repeats up to 99 times.
One tip -- get the tempo as close as possible, then activate the 'AB Qtz' function from pg 59 of the manual,
.