CDI contrat lissee for staff working in horticulture

We run a garden nursery in Normandy and have had staff working for us as seasonal workers the last few years in spring and autumn.  We have one member of staff who would like a CDI with us but due to the nature of the work we do not have enough hours or finances to be able to employ him for a normal CDI with regular set hours per month.  We have been advised by our accountants that we can do a CDI intermittent avec  mise en place d’un repos compensateur de remplacement, setting out his planned hours per month when we are busy he would work more hours and then when we are less busy and closed to the public he would recover his hours in time off instead of extra cost to us.  However to put this in place our accountants suggest we need to set up an additional accord collectif for which they are requesting nearly one thousand euros which seems very expensive and unnecessary as we have an accord collectif covering RCR already . They however feel it is necessary to cover additional details in the accord such as :
– Le nombre d’heures supplémentaires qui ne fera pas l’objet d’un paiement mais pourra être remplacée par un repos compensateur. Il peut par exemple s’agir de l’ensemble des heures supplémentaires effectués par le salarié ou d’une seule partie,
 – Les modalités de choix des dates pour la prise de repos,
 – Le caractère obligatoire ou facultatif de recourir à ce repos compensateur de remplacement, c’est-à-dire déterminer si le salarié est libre ou non de choisir entre le paiement de ses heures supplémentaires ou le recours au paiement, 
– La durée de validité du repos compensateur,
 – La forme du repos compensateur, cela peut par exemple être une réduction d’horaires sur les jours travaillés ou des jours de congés supplémentaires, 
– Le délai de prévenance de prise dudit repos.

My questions to you is can I set this up myself with my member of staff without incurring the fees for the accord -do I need any official legal paperwork or could I not simply add to the contracts the above details which perhaps if necessary the accountant could write for us at a reduced rate to what they are quoting for the accord?

Can I then continue to declare payslips through the MSA if we are using CDI intermittent (we currently use the online TESA monthly declarations for our seasonal staff).
Thank you