It is not a Sylk format restriction, but OWA_SYLK package.
Below is an example of a modified (manually) SYLK file, generated by OWA_SYLK package, where 2nd, 4th and 5th columns are in numeric format. You can modify OWA_SYLK package to do it automatically.
ID;ORACLE
P;FCourier New;M200
P;FCourier New;M200;SB
P;FCourier New;M200;SUB
F;C1;FG0R;SM1;G
F;C2;FF0R;SM0
F;C3;FG0R;SM0
F;C4;FF0R;SM0
F;C5;FF0R;SM0
F;R1;FG0C;SM2
C;Y1;X2;K"ID"
C;X3;K"EMPLOYEE"
C;X4;K"SALARY"
C;X5;K"COMMISSION"
C;Y3
C;X2;K7566
C;X3;K"JONES"
C;X4;K2975
C;X5;K""
C;Y4
C;X2;K7698
C;X3;K"BLAKE"
C;X4;K2850
C;X5;K""
C;Y5
C;X2;K7782
C;X3;K"CLARK"
C;X4;K2450
C;X5;K""
C;Y7
C;X1;K"Totals:"
C;X4;ESUM(R3C:R5C)
C;X5;ESUM(R3C:R5C)
F;W1 1 7
F;W2 2 4
F;W3 3 8
F;W4 4 6
F;W5 5 10
E