Hello, @Mr Milton.

As of this time, manually adding the Serial Number in the Sales Order and in creating an assembly is not available. Once you turn on the Serial tracking feature, it s enabled for every item in all transactions. Also, you'd be prompted that the serial number you've entered does not exist in your inventory. 

I can see how it would be beneficial to you to have the ability to upload your signature electronically. I'd personally forward this to our Product Development Team for future product feature updates.

Get back to me here if you have other questions. I'm always here to help.