If you type the command
3dinfo -verb stats.Imagination001_REML+orig
you will get a screen printout of the information in the dataset header. The information for each volume (=sub-brick) in the dataset will include the type of statistic it is (t or F), and the degrees of freedom.

With that information, you can convert the voxel values from statistics to nominal p-values via 3dcalc. For example, if you find that volume #4 is a t-statistic with 27 degrees of freedom, you can convert it to a p-value with a command like
3dcalc -a stats.Imagination001_REML+orig'[4]' -expr 'fitt_t2p(a,27)' -datum float -prefix brick4p
There is no command to convert all the volumes directly to p-values -- just one at a time, as in the example above. Since you seem to be new to AFNI, I'll mention that volume counting in AFNI starts at 0 -- the first volume in a dataset is #0, the second volume is #1, and so on.

You might want to consider using the expression '-log10(fitt_t2p(a,27))' to save the negative of logarithm (base 10) of each p-value. That way, smaller p-values become large output values, and so they behave as you might expect when thresholding in the AFNI interface.