Can we make a macro for begin{frame}[t]end{frame}?

If you are not into the underlying LaTeX mechanics and would like to seek for a simplistic way, you can just save your command into a file (e.g. cmd.tex) and declare a macro to input it.

cmd.tex

begin{frame}[t]end{frame}

main document

documentclass{beamer}

begin{document}

newcommand{newframe}{input{cmd.tex}}

newframe
newframe
newframe
newframe

end{document} 

Edit: making use of files

As other answers have pointed out, there is no easy way of defining a macro as end{frame}begin{frame}[t], because of LaTeX's parsing mechanism. The (only) way to escape from this dilemma is to make use of files, as I have pointed out before. To support what you want, we can design a (crazy) workaround:

1. Save your TeX source code to file1.
2. Suppose the macro your designed is framesep. We read file1 line by line and replace framesep to end{frame}begin{frame}[t]. The result is saved as file2.
3. Read file2.

The code is shown as below.

documentclass{beamer}
usepackage[T1]{fontenc}
usepackage{expl3}
usepackage{xparse}
usepackage{fancyvrb}
usepackage{datetime2}

begin{document}

begin{VerbatimOut}{jobname.myverb1}
begin{frame}[t]
frame1
framesep
frame2
framesep
frame3
end{frame}
end{VerbatimOut}

ExplSyntaxOn

% replace this regex...
str_new:N l_framesep_str 
str_set:Nn l_framesep_str {framesep}
% to...
str_new:N l_framecmd_str 
str_set:Nn l_framecmd_str  {end{frame}begin{frame}[t]}

ior_new:N l_infile_ior
ior_open:Nn l_infile_ior {jobname.myverb1}
iow_new:N l_outfile_iow
iow_open:Nn l_outfile_iow {jobname.myverb2}

cs_generate_variant:Nn regex_replace_all:nnN {ooN}

ior_str_map_variable:NNn l_infile_ior l_tmpa_str {
    regex_replace_all:ooN {l_framesep_str} {l_framecmd_str} l_tmpa_str
    iow_now:Nx l_outfile_iow {l_tmpa_str}
}

ior_close:N l_infile_ior
iow_close:N l_outfile_iow

ExplSyntaxOff

input{jobname.myverb2}

end{document} 

Note that this can also be done by calling a pre-processor before compiling your TeX source. For example, you can have a Python script doing the replacement first and then calling latex.

If your aim is to substitute the combination

end{frame}
begin{frame}[t]

with, say

breakframe

then the answer is no, you can't.

As beamer is implememented, it needs to absorb the complete frame code before processing it for making overlays and all other things. So LaTeX needs to see an explicit end{frame} not buried into a macro. After processing begin{frame} it looks forward until it finds end{frame} doing no macro expansion, but simply absorbing tokens as it goes.

Yes, one might think to set up the presentation like

startframes
...
breakframe
...
breakframe
...
breakframe
stopframes

but, unless you're wanting to absorb the whole presentation in one big swoop, the final part should be something as described above.

Is this so different from

begin{frame}[t]
...
end{frame}
begin{frame}[t]
...
end{frame}
begin{frame}[t]
...
end{frame}

to go into the burden of rewriting code already working?

A good text editor can spare you the typing and produce

begin{frame}[t]

end{frame}

with the pression of a couple of keys, also placing the cursor in the blank space.