Purchase Invoice vs Gen. Journal vs GL Entry

Hi Guys,

I`m working with NAV 5.0.
I`m traying to add one decimal field to Purch.Invoice Line whitch is need to be transfer to G/L entry table.
I dont`t care about account no.
Also i need to add this field to Gen. Juornal Table, becouse sometimes posting is beeing done only from Gen. Journal.

Resoult of both posting should give me value of my decimal field in GL Entry Line.

What I need is (i think) modify Codeunit 12 Gen. Jnl.-Post Line and Codeunit 90 Purch.-Post.
Codeunit 12 is quite easy to modify and works properly.

I try to use TRANSFRFIELD function, wich is write (in nav standart) anytime before Codeunit 90 run Codeunit 12 with wariable type record, but it doesn`t work.

Maybe i don`t understand Purch. Posting Codeunits.
Please give me some advice.

regards
Wojtekm