Hello Swapnil,
Only alternative is to manage the scenario through different measuring points at the time of filling at external stations.
- Diesel Qty (Ned not be a counter, because you can take care of the total in custom reports)
- Diesel Cost
- Distance meter reading if needed
Obviously these many (and more as per your requirement) measuring points are needed to be created on each vehicle. You can maintain the External station codes in the Catalog1 (valuation code in any one of the measuring points preferably in the Diesel Qty) so that while creating the document you can select the external gas station.(Ignore if not feasible).
Now you need to develop an integrated report combining the in-house and external readings in ABAP. It is understood that the external measuring documents will be linked by fields Vehicle no. (means Measuring point) and the Date of Measuring document.
These are guidelines to address your issue. You need to develop it further.
Best of luck
KJogeswaraRao