Is there any library to import .skp file to Unity at runtime?
Yes and no. There is a C sdk that allows you to read/write .skp files natively. Unity is C#, so you could create/find/purchase a C# wrapper that wraps the c sdk in a managed library, which you then can call from your other c# unity scripts.
I don’t know about at runtime, but at author time Unity isn’t up to date with skp support. You may need to save as to an earlier version when doing the at runtime loading tests.