Hi Praveer,
The solution of this lies in how you will be accepting the input. If you are using FMs, you will define some table which will accept the values. That table may not have any type associated with it to make it dynamic in FM but then you can not use this to map the properties in SEGW transaction (since there are no fixed properties defined).
If the table is having some defined type, then you would be having all those fields declared. As Ashwin mentioned, you have to choose all the possible fields while selecting the properties in SEGW. This way you will be passing values to only those fields which are required as per your business logic.
Regards,
Ekansh