Hey TakaYoshi,
Great question. For what you're asking, just to confirm you're saying the feature layer has each individual floor, and each floor feature layer has an attribute tied to its use?
If so, I just ran a test to confirm this workflow. First off, we do have the "Spaces" import in Plans, under our Development tab. This allows you to bring in building footprints of new potential developments into the model. However, this only works for one floorplate, aka the bottom floor, as you can choose the building type that applies to said building footprints you upload. Also to note as well, you can only choose one building type for all of the footprints to upload to Urban.
With your question, loading in different floors, I tried running two floors (Floor 1 and Floor 2) to see what would happen, and both building types were located on the ground, intersecting with each other, so that wouldn't be a recommended workflow.
However, there is something you could do here, albeit a bit of work. If in your feature layer of all the buildings, you have exactly similar buildings (i.e. you have buildings that go 1st floor retail, 2nd floor office, 3rd floor office), you can select those in ArcGIS Pro and separate them as their own feature layer, then only upload the bottom floor to ArcGIS Online. Then, in Urban, create a Building Type based off of whatever the building is, choose the feature layer you uploaded and choose your personalized Building Type.
To be fair too, that could be a bit of work. I would also recommend bringing in the feature layer as a Context Scene, or a layer that can be used to help derive more context to your study area. The feature layer you have would be perfect to trace out in Urban, and also know how tall each floor would be, what the use will be, etc. Then, when you're done with all of the building, you can remove the context scene from your Plan!