Given that your qualitative data will almost certainly involve a small sample, there will be no hope of useful generalizability, So, I would recommend concentrating on the traditional qualitative approach of "purposive sampling" to get a meaningful set of cases for your purposes.

Can you say more about why you want to go beyond snowball and network sampling?

Thank you Dr. Morgan for your advise. Currently we are reaching a certain group of MSM through our HIV prevention interventions. However, results from a recent study indicates that there are some MSM, who cannot be reached through traditional intervention model (i.e. through peer educators). We want to explore these groups of MSM. Typically, MSM are recruited as research participants using snowball sampling. RDS was successful as well to recruit research participants. My question is , whether we can include other techniques to recruit these hidden population?

After recruiting the research participants, we will utilize a blend of qualitative data collection techniques, such as in-depth and key-informant interviews, focus group discussions etc.

If you have some idea about the characteristics of those in the hidden population (i.e., they are not completely unknown), then that can serve as your recruitment criteria for the purposeful sampling. I know that is not a very explicit answer, but I'm not certain whether there is a better approach.

One place you might look is the book: Mapping Social Networks, Spatial Data, and Hidden Populations (which is a volume in the Ethnographer's Toolkit). 

One unconventional approach could be utilization of social networking apps such as Whisper or Tinder. In both platforms, it is possible to recruit hard to reach population.  However,  at the same time it is hard to convince someone from the target population to participate.  I myself used/is using similar platforms for exploratory pilot study.