a) I want to add in Optical Input to my home theatre system.....
Is that any shop provides such service? (Modify the home theatre receiver & add in Optical input)

b) Home theatre's sound quality depending on the receiver or speaker? If I use a good speaker but lousy receiver, is the sound quality still good?

Thanks.

Both speaker and receiver play their role in sound quality.

I think, what TS should have asked, between the two, which has more important
role, such that, if a person can only pick 1, should he go for receiver or speaker?

your amp