So far I know support Nester the *.tpr, VirtuaNES the *.vtp and Nestopia the *.tp extension.

Nestopia + Family BASIC (J) (V3.0) as example

To save the .TP file type:

First go to Machine > External > Tape Recorder > File...
and type a new filename which will be selected.

File > Open...
and select the Family BASIC (J) (V3.0).nes file

If you have BASIC listing you can copy the content and paste the text (F12)

SAVE "filename" {don't press ENTER yet!}

Select Machine > External > Tape Recorder > Record

Now press ENTER!

When the flashing cursor reappears

Select Machine > External > Tape Recorder > Stop

***

To load the .TP file type:

First go to Machine > External > Tape Recorder > File...
and select the .TP file.

File > Open...
and select the Family BASIC (J) (V3.0).nes file

LOAD "filename" {don't press ENTER yet!}

Select > Machine > External > Tape Recorder> Play

Now press ENTER!

When the flashing cursor reappears your saved program should be restored

Type RUN