There is no hardware adaptor for trxla's and zfr300, only for the Trx900AA and Zfr100.
About the ZFT300 from Zaxcom website.
"The Zaxcom ZFR 300 (Zaxcom Flash Recorder) is the same size as Zaxcom TRXLA2 wireless transmitter and can accept standard lavaliere microphones. A stereo ZFR can accept two lavaliere microphones, or two unbalanced microphone inputs."