There are two types of electronic signatures: digital signatures and e-signatures. e-signatures are in most cases just stamps. EchoSign workflow creates form fields where its e-signatures are placed. For those you can select text that appears as your signature in the "Place Signature" selection in the Fill&Sign tool. Digital Signatures are placed in special unsigned signature fields. If this is what you mean you can create a custom appearance with your initials. First you need to create an image file that contains your initials (could be just your full signature) and then follow workflow when you click "New" in the Appearances section in Edit->Preferences->Signatures->Creation and Appearance->More dialog.