Linking different models

As long as the objects are permitted in the target model you can copy objects and paste them as occurrence copies in any other model. Occurrence copies insures that the new objects that appear in the model are just references to the original object and not new objects themselves.

I would highly recommend taking the objects from each model type and creating a Library group structure to house the definitions of these. Most org and system objects are fairly static and governing these via a Library makes life a lot easier for modellers as they always know where the 'source of truth' is for objects of these types. As Rick mentioned, you can then copy / paste or occurence copy these objects into whatever models support the object types that you are using to capture business process.