Or must I copy and paste each time?
If it's for corporate use, there are tools that can manage this from the mail server (see for example this one).
If it's for your personal mail, there's this App for a dollar that will take your details straight from the iPhone's settings.
To be able to compose a multiline signature that'll apply to all your messages:
Additionally, you can also use:
And compose multiple keyboard auto-expanding shortcuts. By using clever shortcut tags (e.g., l1, l2, etc.), you can quickly make multiple lines of text for a seemingly multiline signature.