This kind of falls into the realm of possible but not built to be automated in the software.
For the first part, Lighthouse Studio does not incorporate utility balance when generating designs, so there is no way to set priors before creating the design. The second gotcha is that when you create a CBC design in the software, the design has a unique ID. This ID is recorded in the respondent's data record. Only one design can be present in a survey at a time, so if your first 100 respondents saw design 1234 and the second 100 saw design 5678, the software will not be able to analyze all 200 people together by default because they saw different designs.
You can technically get around this by changing the design mid study, then combining your two designs into a single design and changing the design ID and version in the data, but it would be a pain.
The second part about combining levels together is again doable, but it's all or nothing. Designs can be exported from Lighthouse Studio to a CSV spreadsheet, modified where you change all the levels from, say, 4 to 3, and re-imported back into Lighthouse. You'll have to deal with the design ID issue mentioned above because importing will create a new design, but the answers will all still match up since you are only changing one attribute.