I was about to sign up but didn't know what way round to do it, I want my own free Domain but want to add a sub domain for the site I am building for someone else (they already have their domain). I will want to build my own site later on & a couple of other small ones too, meaning....

Main - me.co.uk
Sub - them.co.uk
Sub - others.co.uk

Can this be done & if so how?

Pretty please & thank you in advance.