The original sentence has a simple, if wordy, structure
Fetch [verb] the transformations [direct object] which need to be concatenated [adjectival clause modifying direct object].
Your suggested alternative is arguably right with a slight modification
Fetch [verb] the to-be-concatenated [adjectival infinitive construction modifying direct object] transformations [direct object]
While the verb phrase to be concatenated can be used without creating a single term by using a hyphen, it would be very clumsy if used as an adjectival phrase before the noun it modifies. When used in that fashion, the verb fetch is followed by another verb form to be that seems to be veering in a different direction. While it makes sense when complete, it is a bit hard to follow. In print, with hyphens, it is a little easier to immediately understand.
However, Peter Shor's suggestion of leading with the noun is much easier to follow.