It's possible to import another BASIC source file, using an IMPORT statement as follow:

import "some_file.bas"

Assuming there's a "some_file.bas" file at the working directory on disk, this statement will load it at the loading pass, just as if the content of "some_file.bas" was written at where the importing statement is.

Read the Customizing an importer page to get information about how to customize an importer.

Read the Module (namespace) page to get information about importing a module (using a namespace) with the IMPORT statement.