It is very likely that you are stuck, and will not be able to estimate interactions with CVA.
Unless you plan ahead in CVA in the design phase of the study, you will probably not have enough combinations of products shown to respondents to support estimates of interaction effects. I'm assuming you used the approach of 1 questionnaire version in CVA? But, if you used multiple questionnaire versions in CVA (a feature of the newest CVA version), you might be able to do this, but it will require CVA/HB estimation (and re-organization of the design file, as described below). Yes, I said CVA/HB estimation. CVA has an HB module ($1K more to purchase).
The way to do interaction effects in CVA PRIOR to fielding the study, is to plan ahead of time which 2 attributes will be involved in the interaction effect (I know, harder said than done). Then, instead of specifying these as two attributes with 3 levels each, you should specify these as a single collapsed attribute with 9 levels.
Then, during estimation (we prefer CVA/HB rather than CVA + default OLS), you can estimate as all 9 levels (perhaps with additional constraints between selected levels within those 9 that have known order). You get one attribute with 9 utilities, not 3 attributes each with 3 utilities.
Then, if after fielding the study you don't believe that there is an interaction effect that is strong between those two attributes...with additional work you can re-organize the design file (saving a copy of the original study to a new study to work in) and separate the attributes back to two 3-level attributes for estimation. This saves degrees of freedom during the estimation and is a more parsimonious model.
Of course, CBC software makes the estimation of ALL potential 2-way interaction effects possible because it typically manages dozens or hundreds of versions of the questionnaire design across people. This means lots of combinations of product concepts have been seen across the sample. Via logit, latent class, or CBC/HB estimation, interaction effects are possible in CBC.
But, in CBC, if you only field 1 questionnaire version (not typical practice), you'd probably be in the same bad situation with respect to interaction effects as a 1-version CVA study.
BUT, if you use the newest CVA's capability of fielding 10 versions of the CVA questionnaire, then you are probably in very good position to estimate an interaction between any two 3-level attributes in your study. Again, reorganization of the design file to collapse 2 attributes into 1 with 9 levels...plus CVA/HB estimation is needed.
Please call in to tech support if any of this is unclear, or if you need more details regarding how to save a copy of your study that recodes the attributes as one collapsed attribute rather than 2.