Once you imported a custom room in Spoke you have two solution to customize the skybox before importing it in Spaces.
1) You can either play with the parameters of the default Skybox of Spoke
2) Or, if you want a custom one, the steps with Blender would be the following:
Open your 3D room with blender
Create a sphere in the same room
Scale the sphere to be all around the 3D model
Create and apply a new material to this sphere
Open shader editor and add a new texture with sphere projection
Select the 360 image you want to use
Export to .glb and proceed as usual in Spoke
Note that you can achieve the same thing with another 3D software if you are more familiar with 3ds Max or Maya for example.