Thanks Terry i appreciate your help. Now i want to show the full code that uses the solution provided by Sergey. some misunderstanding by others.

*lcInst <== this is the var that keeps filter
*this.lcfield name of the field
*lcvalue value to filter
IF EMPTY(lcInst) &&no previous filter
 lcInst = "set filter to INLIST("+ THIS.lcField +",'"+lcValue+"')   "
 ** 3 spaces to the end ...
ELSE
 CAD1=ATC("INLIST("+this.lcfield,lcInst) &&find the inlist for field
 IF cad1 != 0 &&found inlist, add lcvalue to it
  CAD0=SUBSTR(lcInst,1,CAD1-1) &&get the start of filter
  CAD2=SUBSTR(lcInst,CAD1,LEN(lcInst)) &&get second part of filter
  CAD3=SUBSTR(CAD2,1,AT("   ",CAD2)-2) &&get inlist code to be updated
  CAD4=SUBSTR(CAD2,AT("   ",CAD2),LEN(CAD2)) &&get after inlist code
  CAD=CAD3+",'"+lcValue+"')" &&merge
  lcInst=CAD0+CAD+CAD4 &&new filter ok ;)
 ELSE &&create a new inlist
  **if is a new inlist, note that 3 spaces are added to the end
  lcInst=lcInst+" AND INLIST("+this.lcfield+",'"+lcValue+"')   "
 endif
ENDIF
**this works perfectly with previous instructions that uses and so, this **solution helps to add new filter by mixing "and" with "or"
**and.. i needed to use filters due to this form opens readonly any table to **let user be able to send to excel or whatever... Thank all.

I never used before the button that specifies wich message is the solution, i found it today but i mark wrong message. Sorry about that.