Can Moderators set up sticky post?

Can Moderators set up sticky posts? And if so, how do you change a pre-existing post to a sticky?

Yes they can
Admin Panel>General>Categories and Forums

Click on any forum's name
then click on permissions tab
Advanced Option
For moderators, check the box of Create a sticky message, Hit save.

Then whenever a moderator posts a message, below the message where you type there is a option of
Post Topic as: Check Sticky instead of Normal.

Hope this helps.