To create a wiki post you simply mark an existing post as wiki-editable.
This is a POST level setting, so be sure you are looking at the POST menu, and not the TOPIC menu!
Open the post wrench menu at the bottom of the post (it’s hidden behind the …).
Select “Make Wiki”
Prerequisites for wiki creation
Staff (admin or moderator) and Trust Level 4 (TL4) users can convert any post into a wiki post
Non-staff users TL3 or lower can convert their own posts into wiki posts when they meet the following requirements:
The user must meet the
min trust to allow self wikisite setting requirements (TL3 by default)
The post must be converted into a wiki within the edit windows specified in the
post edit time limitsite setting that is relevant to the user’s current trust level:
Additionally, staff can configure categories to wiki by default by editing the category and enabling
Make new topics wikis by default