All you need is a 1/8" Mono Male to F-Connector (Coax) Female Adapter. Radio Shack sells a gold plated adapter - #278-267. Then any RF-Modulator such as the Philips ph61156 will give you a composite input through the external antenna jack. Here is the manual for the PH61156: http://www.p4c.philips.com/files/u/us2-ph61156/us2-ph61156_dfu_aen.pdf
Enjoy - Brian