UTILIT99 Variant with custom messages

Hi dear experts,

how can we insert custom messages to the variant UTILIT99?

Do I have to copy the function module ISU_UTILIT99  to a custom FM ZISU_UTILIT99 and in there insert the new custom message error numbers?

If so which table holds these messages? (eg. AJ 282)

Also where do you set the relationship between the variant UTILIT99 and the FM ISU_UTILIT? (to change it for the new custom FM)

Thanks for your help

Regards,

Sergio