How to convert U => Seq[T] to Stream[U=>T]

Question

I'm trying to solve this problem. I want to implement this function in Scala

def toSeq[U,T](f: U=>Seq[T]): Stream[U=>T] = ???

I guess the solution should make use of Stream, but I don't know how to solve this.

EDIT

I will try to describe the use case that lead me to this problem. I am creating a DSL to express extractions of data from different datasources. The DSL allows to define extractions like this (U is the source type and T is the extraction type)

type Mapping[-U, T] = U => T    
trait Source[T , U]
case class SingleValueSource[T  U](mappings: Seq[Mapping[U, T]]) extends Source[T, U]
case class OptionalValueSource[T , U](mappings: Seq[Mapping[U, Option[T]]]) extends Source[T, U]
case class SeqValueSource[T , U](mapping: Mapping[U, Seq[T]]) extends Source[T, U]

I want to be able to combine all this mappings to build new Source, for example

def addMapping(source: Source[U,T], mapping: Mapping[U, T]): Source[U,T]
def addMapping(source: Source[U,T], mapping: Mapping[U, Option[T]]): Source[U,T]
def addMapping(source: Source[U,T], mapping: Mapping[U, Seq[T]]): Source[U,T]

The function I am asking came up when I was trying to implement

def addMapping(source: SeqValueSource[U,T], mapping: Mapping[U, Option[T]]): Source[U,T]

But maybe there's a better way