How to convert datatypes?

TeX is a macro language, it does not have any other types than strings. (There are typed registers, but they are not relevant here) The problem is not that DTLfetch "returns" a string, it is that DTLfetch is not expandable.

Non-expandable macros like DTLfetch do not expand to a value, but sometimes they insert some results in the current list. That is why you can use see the result from DTLfetch in your document, but it does not work inside FPeval.

To fix this, you can use a variant DTLfetchsave of DTLfetch which egreg has written for retrieving substring of `DTLfetch`. It saves the result in a new macro, which can then be expanded (similar to FPeval itself):

begin{filecontents*}{students.csv}
name,grade
Joe Bloggs,2.7
Jane Doe,1.0
John Smith,3.0
end{filecontents*}

documentclass{article}
usepackage{datatool,fp}

% From egreg at https://tex.stackexchange.com/questions/335483/retrieving-substring-of-dtlfetch
newcommand{DTLfetchsave}[5]{%
  edtlgetrowforvalue{#2}{dtlcolumnindex{#2}{#3}}{#4}%
  dtlgetentryfromcurrentrow{dtlcurrentvalue}{dtlcolumnindex{#2}{#5}}%
  let#1dtlcurrentvalue
}

begin{document}
  DTLloaddb{students}{students.csv}
  DTLfetchsavegrade{students}{name}{Jane Doe}{grade}
  FPevaldoublegrade{2*grade}
  doublegrade
end{document}

I also discovered xDTLassignfirstmatch in datatool that seems to work the same as DTLfetch, with expansion, and works great for me!

I had the same issue when trying to use data from a (CSV) database for plotting single values with TikZ/PGFPlots or for printing values using siunitx.

With this post I found out how to store all values from the database as (expanded) LaTeX variable named after the respective key:

documentclass[tikz]{standalone}
usepackage{siunitx}
usepackage{filecontents}
begin{filecontents*}{mydb.csv}
key;value
mykey;1.0
end{filecontents*}

% Relevant code below
usepackage{datatool}
DTLsetseparator{;}
DTLloaddb{mydb}{mydb.csv}

begin{document}
DTLforeach{mydb}{Key=key,Value=value}
{%
   cslet{Key}{Value}%
}
DTLfetch{mydb}{key}{mykey}{value} works
% while $num{DTLfetch{mydb}{key}{mykey}{value}}$ fails
mykey prints the value for mykey, $num{mykey}$ also works
end{document}

However, loading multiple databases with similar keys would re-assign the same variables.

As a quick fix, I made DTLFetch assign the values to variables named after a concatenation of database name and key and wrote a function to parse the latter:

documentclass[tikz]{standalone}
usepackage{siunitx}
usepackage{filecontents}
begin{filecontents*}{mydb.csv}
key;value
mykey;1.0
end{filecontents*}

% Relevant code below
usepackage{datatool}
DTLsetseparator{;}
DTLloaddb{mydb}{mydb.csv}

newcommand{getval}[2]{csname#1#2endcsname}

begin{document}
DTLforeach{mydb}{Key=key,Value=value}
{%
   cslet{mydbKey}{Value}%
}
getval{mydb}{mykey} prints the value for mykey, $num{getval{mydb}{mykey}}$ also works
end{document}