ATMega32, PetitFS, writing measurements to file
I think you did not post the whole code, as pf_mount and pf_open are also needed to get started.

First of all - just don't close the file, call pf_write again with another sample.

Second - if you want to append to an existing file - open the file with pf_open and then try pf_lseek perhaps with a very high number (UINT32_MAX). I think that it will just set the pointer to the end of the file (check the error code).