Assigning Packed Type to Unpacked Type

You can use push_back instead:

"
data_in_real = in_data_fq[31:0];
data_in_imag = in_data_fq[63:32];
data_out_real = out_data_fq[31:0];
data_out_imag = out_data_fq[63:32];
"

data_in_real.push_back(in_data_fq[31:0]);