This is possible for pre-registered program webforms. To add a signature line, do the following: 

1. Go to Events > Search for your Event
2. Go to the Registration Information Tab
3. Click the Add button, then Question: 
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4. Select the section of your webform you want this question to appear. Generally, this will be the same section as the item you want signed, ie the Disclaimer or Release Form sections
5. Click the New question button: 
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6. Name your registration question
7. Enter the text you want to see online: 
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8. Click Save
(Optional: Mark it as required)
9. Click Save