Doesn't sound like you care about the column names so this will copy the third column (and its label) of Data1 to the last column in Data2...
ii = Data2!wks.ncols; // number of cols in Data2 copy %(Data1,3) %(Data2,ii); // copy col 3 of Data1 to col ii of Data2 %L = Data1!wks.col3.label$; // get col 3 label from Data1 Data2!wks.col$(ii).label$ = %L; // set col ii label in Data2