With sheep there is no need to roll the barley if you are feeding it in moderate amounts. The sheeps teeth knit close enough together to crack outer husk, that said if planing to feed ad lib then some cracking of the husk is best. As with most changes in diet they need weaned onto the barley to stop bloating. Once the gut is used to the barley then you can feed as much as you like without problems. Normally takes between 2 and 4 weeks to get them onto the barley safely, since your lambs are already eating hard feed then it shouldn't take much more than a couple of weeks.
The problems that can occur with feeding barley are due to the high starch content of the cereal which causes it to ferment too fast in the rumen causing acidification and bloat. This can cause scour, stomach upset and and bad cases it can kill. Try to make sure that if the barley is milled then it is just cracked and no more rather than fully rolled. This slows the release of the starch into the gut and so slows the fermentation. If it is fully rolled then no worries just take an extra week in your feed plan and use smaller increments for uping the feeding level.
One note of caution. Use troughs to feed them to begin with. In the past when I have fattened lambs on barley i would put the feed out then gather all the sheep in to eat. Only once I had ALL the lambs eating would I start feeding twice a day and stepping up the quanity until I could use ad lib. The reason for this is some times in a bulk of lambs a few won't eat until one day they decide that actually they are hungry and then take a gutsful with all the usual symptoms following on! Once you are onto ad lib make sure there is enough feeder space to stop this happening to the smaller lambs as well. Because your lambs have already been eating this process is much easier.
The barley will not be as complete a feed as the pellets so growth will be much slower, a mix of the two is good if finaces dictate that you can't use pellets alone. Some feed comanies produce blends rather than pellets. These can be really good value and provide a similar nutriet balance as the pellets but a fair bit cheaper.