When you harvest a crop, vanilla or modded, you now get a chance to acquire its seed.  Chance is based on farming skill with a default of about 5% with no skill and 33% with lv 10 farming.  You can modify the numbers governing these chances in the config file. 

Config Options

  "BaseChanceToGetSeed": 5.0,    % chance everybody gets
  "ExtraChancePerFarmSkill": 2.0, % added based on farmers skill
  "ExtraChancePerFarmPerk": 4.0,% added if you have the Tiller or Agriculturist professions, counts for each
  "ExtraChancePerDayToGrow": 0.5,     % chance based on how long the crop need to mature, quick crops have a smaller chance
  "ReharvestablePenalty": 0.33     % penalty applied to crops that can be harvested more than once, so corn has 1/3 the chance of giving seeds

The mod uses the crops data from the  Crops.xnb data file, so any mod that adds crop to this file will also work here.

Tested in Stardew Valley 1.11 and SMAPI 1.7