Can I use an airport as an intermediate waypoint in an ICAO flight plan?

Yes.  To do this, tap Maps > FPL > Enter your route into the Route Editor field (FIG 1).  When your plan is ready, send to the Flights page by tapping the Send To button (the square with an arrow pointing up) (FIG 2).  ForeFlight will automatically format the correct routing into the Route field (FIG 3).

 

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(FIG 1) - Maps Page > FPL > Enter route into the Route Editor field.

 

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(FIG 2) - When ready, tap the Send To button > Flights.

 

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(FIG 3) - The route has been entered into the Route field for filing.

 

NOTE:  If you intend to land at an intermediate waypoint listed as an airport, you must file two flight plans.