I sent a second email to frecuenta asking for the transfer and received the reject email again:

"Thank you for your inquiry into Frecuenta/Hilton Honors transfers.
Unfortunately, enrollment and bonus miles are not eligible for transfer to
Hilton Honors. Only miles earned for Mexicana flights are eligible for
transfers. Please check our website: www.mexicana.com.mx/mx2/english/Frecuenta/beneficios/benehilton.asp

Thank you."