Can you move people from one settlement to another?

I think you go into build mode, hover your cursor over them and there should be an option . . I think.

Captain? Anyone?

Yes. In build mode there's an option when you hover over them called move or relocate or something. It gives you a list of your settlements and you can move them to whichever you choose. I've moved a few so that I could balance populations and get supply lines going without using half the population, lol.

Adding to the comment above, yes it is possible to do this in build mode but it will only work if the settler isn't assigned to a resource already in the settlement he/she is already at.

Ah, yeah. Good looking out. forgot that detail.