an electronic signature isn't worth the paper its not printed on.
that said what are you countries standards regarding QA and signatures ...I think you'll find its not allowed..
If you move to project wise program sits ontop of microstation you can hand over a drawing and make it locked to everyone except the drawing approver or signer  and it adds a column to a database showing its been approved and by who.. and with permissions allocated and project flow paths this can do what you want...
Through pdf you cannot realy do what you want as once the pdf is created your engineer cannot add or subtract from it with the acrobat reader program.. he would need to have the acrobate writer program and add a rasterised  version of his signature as a stamp .. you can make custom stamps in the acrobat writer ..
He would also need to security encode this and all your pdf outputs so nobody else could falseify his stamp or remove his stamp...again through acrobat writer this is possible.

Thanks for the reply

From what I have been told, the new Microstation has a new feature so you can convert directly to PDF without loosing quality, line styles or cells. I was also told that if I created a data field script that it would save the data field to be usable in Microstation. I have not been allowed to play with a copy yet but I am supposed to get one by the end of this week.
We use Abobe Professional and with that you can create a digital signature to insert into the file. From there the file is transfered down to our document services department, imaged, and put into the site viewing database.
As far as the signatures being allowed, it was just passed last month that a drawing can be PE stamped electronically now.