I've rated this with only medium confidence because while corps d'état secondaire corresponds to 'fit-out', the term 'fit-out engineer' seems to be used mostly in relation to interior design rather than the actual 'bricks and mortar' aspect of said works. There may well be a cultural difference in how these matters are perceived.

Still, despite other possible interpretations, I think 'fit-out engineer' will get the message across.

Not how the terms I've underlined below match across the two languages.

« Les corps d’états architecturaux ou secondaire » sont eux aussi un nom d’usage pour classer certains métiers du bâtiment.
Ils représentent entre-autre les lots du bâtiment suivant : la menuiserie intérieure, la plâtrerie, la peinture, les revêtements durs et souples…
https://lecoindestravaux.re/top-9-des-termes-du-batiment-que...

finishing trades Mainly joinery, plasterwork, tiling, paintwork and flooring. The completed work may need protection from damage or the weather.
fit-out, fitting-out (USA outfitting) Work in the finishing trades, such as putting up suspended ceilings, fixing prefabricated joinery, laying carpets, etc.
Scott, Penguin Dict of Bldg


--------------------------------------------------
Note added at 14 hrs (2024-05-17 08:35:22 GMT)
--------------------------------------------------

Pour François
"Corps d'état : chacune des spécialités du bâtiment, et ceux qui les mettent en oeuvre : charpente, couverture, plomberie, peinture, etc., sont des corps d'état."
[Dicobat]
These building specialities are divided into two main groups gros oeuvre, the foundations, walls, floor slabs and roof, and seconde oeuvre, everything else that is more or less permanently fixed to the gros oeuvre.

Within the corps d'état secondaires things can get complicated, depending on the contract breakdown, contractors' and owners' habits, so you get further corps with a lot of overlapping subgroups. From my notes :

Corps d'état (trade)
CET - technique - Building services and equipment (BS&E), mechanical and electrical (M&E)
CES – corps d'état secondaires / second oeuvre – Fixtures and fittings (F&F) + BS&E/M&E, fitout, internal works, non-structural works (as opposed to gros oeuvre), building finishing(s) – marché de second oeuvre = building finishing market
CEF – Corps d'état de finitions (F&F)
CEA - architectural = F&F, fixtures and fittings, fixtures and finishings; FF&E, furniture, fixtures and equipment. Fitout. Interior fitout. As such, part of CES .
corps d'états séparés [not = CES] – (built as) a series of separate work packages