This preev.com is exactly what I need but I'll have to do 100's of conversion manually,edit csv and upload it in electrum. Does anyone know by automatic way? Maybe a API of preev.
My recommendation would be to try with Excel or LibreOffice Calc.
You put in column one wallet and the ending comma and on the column two you put a formula, based on, let's say column 3 and the fixed rate you get from preev, bitstamp or whatever.
Then you copy the columns 1 and 2 and you have the equivalent of the CSV, which you can save into CSV or directly into Electrum's pay-to-may. From my test on testnet it works, even if it has some potentially scary extra spaces here and there.
If it's unclear, if you tell me what format you want - Excel or LibreOffice - I'll prepare a sample for you and put it on WeTransfer so you can download it.