Well there's a couple of things that have to be considered
1) Is participant observation (or alternatively, qualitative description) been included in your ethics application regarding methodology and has been given approval?
2) Has this been properly explained to your research participants that participant observation (i.e. that you'll be writing about your observations of them AS WELL as drawing analysis from the interviews) is part of the research study? Do your advertisements looking for participants or explanatory statements include a discussion that explicitly states that participant observation is a component of this study and is this included on the consent forms your participants must sign? Is participant observation explained in a clear and coherent manner so your research participants are aware of what this entails?
If no to either of those, then no, you cannot write about participant observation as per ethics regulations. Your research participants have to be made aware of all components of their engagement with the study, if this has not been done prior to their becoming involved then you cannot include this data. You also cannot add it later on nor amend your ethics approval to include this after having done interviews. You can (potentially) alter the study and amend ethics for *new* participants, but not for those already been interviewed.