I like what Uwe Flick states about qualitative research, which I will generalize here: We are only limited by our own abilities. If you (or if on a team) have the skills to conduct a qualitative and quantitative analysis, as long as there is a suitable research purpose that can be answered with mixed methods, then there is no limits!
If you are talking about extending a qualitative study into a mixed methods study, the most common design for this is an exploratory sequence (qual --> QUAN), where the results from the preliminary qualitative study help develop the content for a survey or program intervention.
The other possible design is QUAL --> quan, but the field of mixed research research has done little to develop this option.