Employ the Import 3D files tool in an ArcGIS Pro version prior to 3.0 and access the created multipatch via a file geodatabase in ArcGIS Pro 3.x.
Create a feature class in a file geodatabase while selecting Multipatch for the 'Type of features stored in the feature class' parameter, and import the .obj file by clicking the Edit tab > Create > model file > Add one or more models.