you can write function to add two numbers and call in qtp.

file-->New-->function Library

Public function add(a, b)
   add=a+b
End Function


Save it where ever u want with add.vbs

Now File-->settings-->Resources-->Add function library--
>click at add(+) button below one button will appear click 
that and add the saved library file(add.vbs).
 
in qtp normal file 

write print add(4,5) and u can parameters as your wish.



add this in

One way is:Dim a,b,c
a=10
b=20
c=a+b
msgbox c

Anotherway:Place the input parameters in these: edit-
>Action Properties->Parameetrs->Here we can add input & 
outputparametrs.
Dim a1,b1
a1=Parameter("a")
b1=Parameter("b")
c=a1+b1
msgbox c

Aliter: Add the values of both a,b of parameters in Action 
Properties.