Understanding Society User Support

I'm doing an undergraduate dissertation using the BHPS data and plan to use waves i through m. Crucially I need to make use of the responses to some questions which are only asked once. "school leaving age" (wSCEND) being my primary variable of interest. Obviously values are only recorded for this variable if the respondent joined in wave i or later, leaving 50,000 of the 60,000 or so observations in my dataset with the value as "not applicable". How can I/can I retrieve the answers to these questions for the respondents who previously appeared in some or all of waves a through h? I also want data on several other variables which are asked just the once.

I'm using STATA