I guess, now you can't get it from ECC. Solman key is used only at the time for Installation/upgrade of SAP and for temporary purpose, only at that time. And then you install the temporary license or the permanant license via saplicense utility or by using slicense transaction
You can get the solution manager key only from a solution manager system.
Find the SAP note
811923 - Generating the SAP Solution Manager key
Refer the following link