Hello Manish,
When we are doing STO PO the Standard Document Type is UB, Instead of Vendor there is supplying Plant.
More over we can do Stock Transfer using MM Route / SD Route
The below options are w/o excise
MM Route - PO (ME21N), GI (MIGO), Gr in receving Plant (MIGO)
SD Route - PO (ME21N), VL10B (O/B Delivery), Vl02N(PGI), VF01 (Invoice) ,Gr in receving Plant with reference to O/B delivery (MIGO)
Instead if you want to do in two step use Mvt Type 303 & 305 in Migo
I feel automation is possible but again there first which route has been configured, MM route will be a little simple for automate but SD route will be a little difficult & there are various challenges that need to be taken into consideration as PO qty is 100 but in stock only 70 available, if there is batch stock then how to pick on fifo basis or lifo basis.
I suggest you need to convince the user on using Standard SAP Process.
Thanks & Regards
Shashank