Answer: interpolate

AnswerCrossword Clue
INTERPOLATEMake insertions.
INTERPOLATEAdd extraneous matter
interpolateInterject (a remark) in a conversation
interpolateInsert (an intermediate value or term) into a series by estimating or calculating it from surrounding known values
interpolateto insert a word or passage in a book or manuscript, esp in order to mislead
interpolateInsert (something) between fixed points
interpolateInsert (words) in a book or other text, esp. in order to give a false impression as to its date
interpolateMake such insertions in (a book or text)