I think you'd need to parse the Cisco configuration files and transform them into XML. Then you'd have to transform them back for loading them onto the equipment. It'd probably be worthwhile to do that if you find XML is the best format for dealing with the others.

You could even come up with a completely standardized XML application for describing network device configuration. You could have a module for each vendor's configuration language, and just call your toXML and toVendorConfig methods after you've set your object to use a particular vendor's language. A name for this language, a DTD, and a website for your language, and you've suddenly got the reference implementation of a new buzzword.

See also Pancho (created by c) which originally supported configuring Cisco equipment over SNMP and TFTP but has templates-based support for the configurations of other vendors' products. It may be a good starting point or at least give you some ideas on how you want to do this. It may even save you implementing your own solution altogether.