I previously downloaded the 2009 ACS 5-year sample, which did not include data on WKSWORK1; but WKSWORK1 is available for 2005-07 in the single-year ACS samples for 2005-07. Question: are the variables that uniquely identify cases (DATANUM, SERIAL, and PERNUM) the same in the 2009 ACS 5-year sample as in the single-year ACS samples for 2005, 2006, 2007? Can I accurately merge data from the single-year samples into the 5-year sample? (If so, I would link MULTYEAR in the 5-year sample with YEAR in the single-year samples.)
While you are correct that DATANUM, SERIAL, and PERNUM will uniquely identify an IPUMS record, the the Census Bureau adjusts the weight values when creating the multi-year samples. Therefore, merging data from a single-year file to a multi-year file would not give you accurate estimates. You could conduct your analysis as you desire by building your own 5 year file from the appropriate single year files.