Just wondering if I can replace an external steer door frame with timber on a brick house.
Welcome to the Bunnings Workshop community @glen71. It's fantastic that you've joined us and many thanks for your question.
It is possible to replace steel door frames with timber on a brick house. First, you'll need to remove the door by unscrewing it from the hinges. Remove the screws from the frame with a power drill and appropriate bit. To remove the jambs, a pry bar will come in handy. If the door frame is welded, you might need to cut out sections to release it. Next, you'll need to purchase a suitable timber jamb and mouldings to cover any gaps. Fix the new timber jamb to the brickwork with Liquid Nails and suitable masonry plugs. Re-install your hinges, door and locking mechanism.
If you wanted to replace the steel jambs because of their aesthetics, you might like to read through this previous discussion: Ugly steel door frames... How to change? by @amclaughlin09. Within it, we discuss the possibility of simply cladding over the steel door frame with timber mouldings.
Please let me know if you need further assistance or have questions.
I haven't personally heard of steel door jambs be used as a structural element in a brick home @glen71. A lintel above would generally be used to support the bricks. However, if you are concerned about this, I would always advise you to employ the services of a qualified engineer.