CanI use existing 110v conduit for adding new 220v outlet in basement?
The conduit fill and how many current carrying conductors would be the issue.
If there is less than 4 current carrying conductors for this conduit adding 2 more will not be a problem with derating standard 75 degree wire and components.
If the fill will allow this could be code compliant.
more than 6 and the derating schedule or fill may be at the limit.
It is ok to run 240v circuits in the same pipe with 120. A 120v circuit has 2 current carrying conductors hot and neutral, a 240v circuit has 2 carrying conductors hot and hot.