1. No, you can't use a non-defining clause like this. It has to go, between commas, immediately after the noun it refers to.
2. These sentences are not especially common (outside of grammar books!) They are rarely used in speech, except in formal situations. In natural, informal speech, native speakers would be more likely to use a fragments like this.
"Mr Smith (pause) - the guy I'm working for (slight lengthening of word 'for' and slight pause and change of intonation to show that you're resuming the main part of your sentence) - is really generous about overtime payments"
This way of talking is actually very common, but may be difficult for non-natives to get right. Otherwise you can use two separate sentences.