You will have to remove the current plastic foresight first - it's just a push fit so easy peasy to get off.
If you get the Gmac silencer adaptor, they also sell a brass pin which you can screw into the adaptor which will act as a foresight. If shooting with open sights, avoid the short and stubby silencers, and go for a Weihrauch (or 'HW') silencer. Not only is it slim and very efficient, but if you ever sell the pistol on you can either keep the moddy for another airgun or move it on for not much less than what you bought it for.
Link for the adaptor and sight pin:
http://www.gmaccustomparts.com/epage...ducts/001-0009
The moderator/silencer you will have to buy in person from a gunshop due to the VCR act. Any gunshop that sells airguns should have one for sale. Or wait for one to come up 2nd hand on here, as they do frequently.