There may be easier ways to make a capacitor symbol from a .SUBCKT but this works for me:
1: Copy (Ctrl C) the .SUBCKT definition for a capacitor from e.g. the manufacturer, an example is shown below:
.SUBCKT EGXF350ELL511MJ16S_20deg-C 1 2
C1 4 2 1.19e-04
C2 5 2 2.64e-04
C3 6 2 3.42e-05
R1 3 4 7.08e-02
R2 4 5 2.99e-02
R3 5 6 1.57e+00
L1 1 3 5.40e-09
This SUBCKT is for a 510µF CHEMICON electrolytic, full data at EGXF350ELL511MJ16S - Aluminum Electrolytic Capacitors | Nippon Chemi-Con Corporation
2: Paste (Ctrl V) the .SUBCKT definition into a QSPICE schematic and agree to auto-generate a symbol.
3: Note the auto-generated symbol is a yellow rectangle, not a capacitor symbol.
4: From section C in Symbols & IP Browser add a suitable capacitor symbol to the schematic.
5: Open the symbol properties for the yellow auto-generated symbol by double clicking it.
6: Highlight the contents of the library file section and copy the SUBCKT definition including the first two characters i.e. PIPE DOT, using Ctrl A then Ctrl C.
7: Open the symbol properties for the capacitor symbol and paste the copied SUBCKT definition into the empty library file section.
8: Also copy the contents of the 1st String Attribute from the auto-generated symbol to the capacitor symbol.
9: Using the drop-down menu change the symbol type from C to X to represent a subcircuit defined in a library.
10: You should now have a capacitor symbol that you can use for circuit simulation, that behaves according to the subcircuit definition.