Could you tell me if the xsl:include and/or xsl:import can include or import
another kind of files?

like .asp, .html, etc..

or just .xsl, .xslt files?

The file extension does not matter. If you generate some
XSLT code with an ASP page, you can use
 <xsl:import href="http://localhost/foo.asp"/>

The result must be valid according to the XSLT spec, as
already mentioned.