If you have the sub-circuit in its own hierarchical page, just copy the hierarchical sheet .sch file to another project, add a new hierarchical page in schematics and point it to this file.
You don't even need to copy the file, you can point the hierarchical page to the original .sch file, but beware any changes to this file can now be done in either of the projects, and will be shared across both projects. So this usually works only for linked projects (sheet with connector, so that both projects have the same pin layout)