providing you have room to use, you can grow a slice that is not mounted or in use. You can do this be deleting another slice, and tacking it on to the slice you want to grow. If you have data on a slice you want to grow, then no you can not grow it. You can modify the table accordingly and then label it to take affect.
You can also use metadevices (SVM or disksuite : same thing just different names) you can concat/stripe slices together, so yes you can enlarge a UFS filesystem as long as its under a volume manager control, without a volume manager, then no.
you might want to take a look at ZFS, maybe it would be something of interest.