I'm not sure what field on IT0002 (Identity) you would base the Seniority Calculation, but I don't think that there is an operation that can calculate it from IT0002, and it would be the same situation if it would be from a field from IT0006 (Address). Perhaps you could ask an ABAP resource to create a Custom Operation, but an easier solution could be to use a Date Type on IT0041 for that purpose. Even then, the standard operations would not give you a result as X years, Y months and Z days, but rather as X years (with decimals), or as Y months (with decimals), or as Z days, so you could use operation NUM= to calculate the number of Service Years from IT0041 and multiply the result by a constant (from V_T511K) whose value is 15208.33 (the constant would be divided by 1000 in the PCR). As for the other part of the formula, "3000X14/12=3500 plus 3500X12=42000" is the same as 3000X14/12x12=42000, or BASIC x 14, so that number could be "hard coded" in the formula or it could be an other constant.
You could then create a PCR that would process the Basic Pay WT, use operation NUM to get the number of years of service, multiply that by the first constant and divide the result by 1000, multiply the result by 14 and multiply the resulting number by the amount (MULTI NAA) and zero out the rate and number (ZERO= RN) before transfering it into your EOS WT.