Add additional params into existing SP

Question

Hello,
please, how can I add additional params: @LastName and @FirstName into existing procedure that starts:
@PIN nvarchar(13)
  AS
DECLARE @v_exist int
set @v_exist=0;
Namely, upon adding additional params:
@PIN nvarchar(13)
@LastName nvarchar(20)
@FirstName nvarchar(20)
  AS
DECLARE @v_exist int
set @v_exist=0;
and adding additional table that comprises columns FirstName and LastName into existing Select statement(referenced table that comprises all possible values of PIN) by creating corresponding relationship, the SP freezes, fails without any output....
Note: SP checks for PIN that is entered into WEB application, in certain T1. If exists, it informs user that PIN already has been entered and new one must be of different type. But, I would like the First and Last name to be checked, too, i.e., in newly
added table T2 I would like LastName and FirstName to be extracted based on entered PIN and, upon that, to have checked these couple LastName AND FirstName in T1 (because, there are rows with missing PIN and fact that result could be more then one line is
not problem because T1 comprises only cca 300 rows... ) 
Tables T1 and T2 are not changeable but reference tables...The data entered via interface goes into some other tables...

Answer 1

Must be because, one person with it's belonging PIN can be entered only once into e.g., T1. Evaluation in the SP checks if entered  PIN is in the T2, which comprises forbidden PINs, LastName, FirstName...etc...But, in some cases i.e., some rows, there
is missing PIN which is to say that I had to involve new table that comprises all possible values for PINs based on which I would like to extract LastName AND FirstName and check it against T2...i.e., check T3.LastName with T2.LastName AND T3.FirstName with
T2.FirstName...
But, like Mr.Sorna advised, without table scripts and sample data ...etc...my thread is too ambiguous...I am afraid...