They were used to the metric system which they used when working for the V-2 at Penemünde. But what unit system did they use when working for NASA? Did they calculate using metric units at first and converting the results to non metric units later?
Everything I've seen from Mercury to Apollo has been in imperial units but that doesn't mean they used metric and converted it all over before handing it over. There is a pretty good article here that talks about the Apollo guidance computer. The computer was programed to use SI units, which is basically metric with a new name, but everything entered into it and outputted from it was in imperial, like in good American fashion. NASA still technically didn't switch to metric for future mission until 2007 (source) so SLS/Artemis will be the first fully metric US missions for NASA.