I have 2 suggestions:
Just have your aliens use the entire planet as office space (intra-galactic admin is no joke).
If your office structure covers the entire surface of the planet then, not only would it be so large as to be effectively infinite to any walking human, but it would also form a closed loop in any direction, so that one could travel indefinitely in any direction without ever finding an ending. (Towers and gangways could just be their way of building on the unique surface of this planet that is used for admin anyway because it is unsuitable for anything else)
As an added bonus, intelligent alien tech building technology that constantly reshapes rooms and furniture to best suit the needs of our office-workers at any given time, means that it would be impossible for your character to find himself in the same room twice, even if he makes several loops of the planet (which would take many decades anyway) as it will have been optimised out of existence.
Have your character wake up in a computer simulation
How they end up in here is irrelevant. (Are they fully immersed, TRON style, or is this more of a Matrix situation? Who knows?) The result, however, is an environment that a computer can easily expand in any direction in a generated "office" style, as required by the author, as far as, and as soon as, the character walks in that direction.
Procedural generation ensures that the environment always remains the same in a specific region, and sophisticated alien tech (made by HumanWare?) can easily handle it with its massive storage. (Although our Alien Admin might have to increase the storage space every now and then if the character gets too adventurous.)