One of the related list in our Deals and Leads module is selecting Products. I am not able to mandate selecting products at the time of lead creation/deal creation or lead conversion to deals as product is an external module and hence Layout rules/validation rules cannot be created, like for example  for a change in a column in 'deal' make 'product' mandatory'.

Is there any custom function that can be written to achieve this functionality/or what is the how to achieve this?