The proper way is to analyse the variable types one by one (using vtypex() or proc contents ) and fix the bad ones.
A faster way is to redefine them all in bulk and let sas do the translation, like:
[pre]
data clean_up;
length varc1 varc2 $8 varn1 varn2 8;
keep varc1 varc2 varn1 varn2 ;
set excel_import(keep=tempvarc1 tempvarc2 tempvarn1 tempvarn2);
varc1=tempvarc1;
varc2=tempvarc2;
varn1=tempvarn1;
varn2=tempvarn2;
run;
[/pre]
Depending on how much clean-up is required, the 2nd way might be sufficient.