There is no import feature available for Host Groups and as this data gets written directly to the database so I dont think there would be any file which we can modify.
However you can still call support and check if they have some workaround for it.

I can import Group Properties for the I-DHCP Enforcer, so i added in a dummy mac address (44-44-44-44-44-44) and it added this line to the XML. If it was just a simple case of updating the XML with the 600 MAC addresses and importing it back into the SEPM this wouldn't be a problem but i can't understand what the rest of the items are in the line,

i.e. what is the <MacAddress _d="false" _i="BEB2A3B00A01300D01BDBFECE7F3232A" _t="1245144919489" _v> part mean?

<MacAddress _d="false" _i="BEB2A3B00A01300D01BDBFECE7F3232A" _t="1245144919489" _v="4">44-44-44-44-44-44</MacAddress>

Or what about even writing these MAC addresses directly to the Database using a PHP page? would this be possible?

any advice? 

Doesn't look like we are going to get much help with this! I've been searching through the database looking for the entries that appear when a MAC address is added to the host exclusion list but can't find anything.

I'm starting to get more and more frustrated. Especially when we had a guy from Symantec visit us tell us that it's a simple enough thing to automate, then when he leaves doesn't accept phone calls or reply to emails surprisingly enough.

If i knew even how the details of the trusted hosts are added to the database i could get a php script written to do this! would help you with your voip problems and me with my printers!

If i can get some kind of solution to this i will create a document for it outlining the procedure, so it will be beneficial for all, i'll also post any PHP scripts/pages which have been useful for me.