PowerShell classes can be defined by commands like scripts, functions, script blocks and imported to the current scope using dot-sourcing.
Such dot-sourced classes can be used for basic operations like instantiation and invocation of members. See the script Test-1.can.use.ps1.
It is not possible though to define a new class derived from a dot-sourced class. See the script Test-2.cannot.derive.ps1.
See also Parser-fails-on-custom-type. The issue is similar. A possible workaround is to move the definition of derived classes to a script and dot-source it after dot-sourcing base classes.
Another workaround is suggested in PowerShellTraps/issues/10.
Define or get the derived class source as a string and use Invoke-Expression.
See Test-3.workaround.ps1.