Importing a string such as "3d456" from a CSV file causes trouble

You can use Import[..., {"CSV", "RawData"}] to treat all fields as strings. Unfortunately that means all fields whether or not they're in quotes. So then you need to use ToExpression to parse specific fields into numbers.

Maybe a little preprocessing followed by a little post-processing?

First, let's contrive some random but reproducible data with of the kind you working with.

SeedRandom[1];
  With[{m = 3, n = 5},
  data = RandomInteger[100, {m, n}];
  Module[{i, j},
    {i, j} = RandomChoice /@ {Range[m], Range[n]};
    data[[i, j]] = "3d456"; 
    data]]

{{80, 14, 0, 67, 3}, {65, 100, 23, 97, 68}, {74, 15, "3d4", 4, 100}}

Now let's pretend that we have read in a CSV file with the above data as a single string.

csvData = ExportString[data, "CSV"]

"80,14,0,67,3\n65,100,23,97,68\n74,15,'3d456',4,100\n"

which looks like

in a notebook. The preprocessing changes the internal double-quotes into single-quotes.

preData = StringReplace[csvData, {",\"" -> ",'", "\"," -> "',"}]

"80,14,0,67,3\n65,100,23,97,68\n74,15,'3d456',4,100\n"

Now we do the conversion to Mathematica data.

mmaData = ImportString[preData, "CSV"]

{{80, 14, 0, 67, 3}, {65, 100, 23, 97, 68}, {74, 15, "'3d456'", 4, 100}}

It's not quite right because of the single-quotes. The post-processing gets rid of them.

postData = Map[Switch[#, _String, StringTrim[#, "'"], _, #] &, mmaData, {2}]

{{80, 14, 0, 67, 3}, {65, 100, 23, 97, 68}, {74, 15, "3d456", 4, 100}}

postData == data

True

Update

The OP asked for a function that would implement the procedure described above. I am posting such a function, but without any discussion of file system functions used or why I made the choices that I made. This update would be better if such a discussion were included, but I simply do not have the time to write it.

csv::fail = "import from `1` failed";
csv::ok = "data imported from `1`";
getCSVData[] :=
  Module[{filePath, str, csvData},
    filePath =
      SystemDialogInput["FileOpen", {"*.csv", {"CSV Files" -> {"*.csv", ".CSV"}}}];
    If[filePath === $Canceled, Return[{}]];
    str = ReadString[filePath];
    If[str === $Failed, Message[csv::fail, filePath]; Return[{}]];
    csvData =
      ImportString[StringReplace[str, {",\"" -> ",'", "\"," -> "',"}], "CSV"];
    Map[Switch[#, _String, StringTrim[#, "'"], _, #] &, csvData, {2}]]