in simulation predictive check mode you can simulate parametrically from your omega matrix of the etas
as for resampling from a covariate database the solution will be to do this step beforehand in R or other tools so you build a template simulation dataset that has the covariate distribution you are after.
so you will have a template data like this:
ID TIME COV1 DV
1 0 resampled value from data base mptyplace holder
that you plug into model simulation note that if you simulate many replicates the covariate distribution will be fixed and will not chagne from replicate to replicate if you want that then
you might want to try other software tools Trial Simulator and or Trial Designer where you can easily resample from a csv file your covariate distribution with options random(random selection) or linear (in order sequential selection)
or just use Phoenix NLME in command line from R and script the data template and iterate