Yeah, a travel agent can do that, and may be worthwhile if you’re using them anyway. At least, if/when I use a travel agent, a feature tends to be that I don’t have to deal much with details.
You can do it on several of the booking sites, but I often have not done that, because typically it’s complicated enough trying to find and book things without also combining them. I have done it a couple of times though for simple trips because it seemed to reduce the overall cost a little bit.
(But to me, that’s one of the pitfalls, is spending hours trying to find the least expensive way to book things.)