It would be astronomically implausible. Knighthood was not merely a profession but a social class. A deeply cherished and protected social class--for a peasant to be elevated to knighthood would be more than a "promotion" or recognition of ability and service, it would be an affront to the social order. There was more flexibility in the early Middle Ages than in the "High" Middle Ages; there may be records of (for instance) William the Conqueror's men being elevated to knighthood from humble origins, but.....not that I know of.

Additionally--being a knight was a tremendously expensive proposition. Arms, armor, a steed, a squire, a second steed. At a minimum. Bannermen and attendants also. The feudal system would make it very difficult for a peasant to acquire that wealth, much less have a sustainable stream of wealth.

Marriage between the classes was quite rare. A wealthy noble woman marrying a peasant is, as far as I know, unheard of.
Adoption...a peasant boy being adopted by his lord, can and did occur. That might be a pathway.

The first example that comes to mind of a peasant becoming a knight....is Joan of Arc. A somewhat unique case.

The second is the Knights Templar/Hospitalier. But even in these extreme cases....I do not know of a peasant receiving the orders.