interpolate

To introduce (something) between other things; especially to insert (possibly spurious) words into a text.

in verse 74, the second line is clearly interpolated, probably by some unknown medieval scribe

When interpolating an explanatory gloss, please enclose it in square brackets to make clear that it is interpolated.

To estimate the value of a function between two tabulated points.

During the course of processing some data, and in response to a directive in that data, to fetch data from a different source and process it in-line along with the original data.

A macro is invoked in the same way as a request; a control line beginning .xx will interpolate the contents of macro xx.