Making built in cupboards into wardrobes

[Floridasunset]

The house we are buying has built in cupboards in 3 of the bedrooms. They are all different sizes but have nothing inside so are just empty spaces.
Has anyone used something like the Ikea pax inside an existing cupboard? I'm not sure if you can just use the internal components without purchasing the doors with it.
Or will we need a carpenter? I'm assuming this will be quite expensive.

The master bedroom has a built in wardrobe with sliding doors. But this looks like it was done later whereas the others are part of the house iyswim. It has been done very badly so we will probably need to start again with it.

Any ideas or experiences?
Thanks

I bought a sliding door kit from Amazon to make my alcove into a fitted wardrobe, it works really well but helped that my dad could help us as he's excellent at diy. In our son's room we had a much smaller space to work with and I got a carpenter in to do it, this was before the one in our room so in retrospect I'd probably have a go at costing it up to do myself if possible!

If there are already cupboards in situ, could you just buy extendable hanging rails and fit them inside the existing cupboards? Maybe check they are deep enough to accommodate hangers first! Although IKEA do also sell hanging rails that are back-to-front rather than side-to-side.

These are cut to size rails - www.screwfix.com/p/smith-locke-round-wardrobe-rail-polished-chrome-1829-x-25mm/1082v

You can buy all the components of the IKEA Pax wardrobes separately, and have doorless frames too.

I would look at the IKEA Algot system instead. Its an easier way to add storage and you arent paying for wardrobe sides when you dont need them.
You can have rails, baskets/boxes/shelves etc.

i.pinimg.com/736x/68/5a/b0/685ab05280e42e58432e6e718cb6849c--ikea-closet-organizer-closet-storage.jpg

I am about to use this in my sons room but havnet yet done it but the Algot system does get rave reviews and there are some good US blogs with details/suggestions on how to use it best.

Thanks everyone for the ideas. I like the look of the algot system as it will give us shelves and drawers not just rails.
Good point about them being deep enough, that hadn't crossed my mind! I will definitely check first

I think the Algot could work even with shallow depth as you could have your hangers on the diagonal.
There is a planner on the website - note to self.