I've found a backfield coach in his final year. I thought it was possible to 'nab' these off other teams if they're in their final year of contract? It doesn't let me as all I can sign him as is an Off/Def coordinator or HC?!?!

Correct....
Can only sign positional coaches from another team by promoting him. Unless...... he gets fired by the hiring of a new coach and then some other owner doesn't sign him before you do.
So, if he is in his final contract, your choices are to either promote him or wait for him to hit the market

i think you can only poach that coach if you make him at a higher position like Off/Def Coordinator or head coach.

Can yoy only do this a certain time of year. I don't even have the option to offer a coach a contract. He's in his final year as a position coach and I want to make him my head coach. But there's no contract button.

If the coach you want to promote has already signed a contract that season, you cannot offer him a promotion. you must "fire" him first by signing his replacement .. then tendering him as an unemployed coach .. but everyone else has an opportunity to sign him as well.