Making hybrid crops in Harvest Moon: A Wonderful Life isn't too difficult. You just take your fruit or bags of seeds and talk with Tartan, a two-headed plant living in Takakura's house. When you talk to Tartan it may give you an option to enhance a plant species. At that point just give it the two plants you want to mix together and Tartan will do the rest.
To aquire this odd talking plant, you must be in Chapter 2 of the game. Go into Takakura's house and you might jump into an event where Tartan mysteriously takes root in Takakura's bedroom. After that the plant will stay put in the corner of his house. When Tartan first sprouts, it hasn't had any time to establish a good relationship with you. If you want Tartan to mix plants, you have to become friends with it. Talk to it every day and after a season or so Tartan will give you the option to mix plants.
There is a restriction to the types of plants you can mix together. You can't mix together a tree plant with a crop plant. Sorry, there's no Tomapple Trees in this game Tartan won't mix crops with a 100% sucess rate either. It may fail several times before the mix results in a hybrid crop.
Since each type of crop has a rank (B, A, or S) Tartan can create crops with ranks. If you mix together a B-rank crop and an A-rank crop, your sucessful result will also be an A-rank. How handy!
Besides plants + plants, Tartan can enhance regular crops by using the flowers that sprout around Forget-Me-Not Valley. If you bring Tartan an Upseed Flower (Winter season) and a bag of seeds, it can mix the two together into an S-rank seed! The flower rank-ups only work for crop seeds and not with tree seeds.