Message no. GB175 on create a substitution step on Asset Accountig Posts

Helllo again, friends!!

I'm trying to create a substitution step on Asset Accountig Posts but I have the following error:

You may not substitute any fields for this Event

Message no. GB175

I must do as below:

In SM30 -> Change table VWTYGB01.

But witch Boolean Class may i have to change and what tables should I enter?

I need to change the HKCONT field when the fields BUKRS, BWASL and AFABE provide specific (fixed) values.

PS: I think that I can do this with Exit AINT0002, but I´m not try it yet. I´m trying to do this first by substitution rules.

Thanks in Advanced for any help!!

Rgds,

GLippmann