The quickest and easiest way is to adjust your table files. You'll need to make the Shivan weapons player usable.
I'd do it myself, but I can't do attachments.

Download VPView from the HLP FreeSpace downloads page, extract the weapons.tbl from root.vp and adjust the Shivan Laser entries.
Don't play with the ships.tbl for the moment, although that will allow you to let players fly the other Shivan ships.
Anyway, once you've extracted the weapons.tbl replace the Shivan Light Laser entry with this:
$Name: Shivan Light Laser
+Title: XSTR("Shivan Light Laser", -1)
+Description:
XSTR(
"Standard Issue
Level 3 Hull Damage
Level 2 Shield Damage", -1)
$end_multi_text
+Tech Title: XSTR("Shivan Light Laser", -1)
+Tech Anim: Tech_Subach_HL-7
+Tech Description:
XSTR(
"The least powerful of the Shivan lasers.", -1)
$end_multi_text
$Model File: none ; laser1-1.pof
; The following fields (preceded by @) are only required for laser (the 3 ellipse
; thing) rendering, which requires that "Model File:" be none.
; These are the same values that used to be hard-coded:
@Laser Bitmap: laserglow01
@Laser Glow: 2_laserglow03
@Laser Color: 250, 30, 30
@Laser Color2: 250, 30, 30
@Laser Length: 7.0
@Laser Head Radius: 0.90
@Laser Tail Radius: 0.30
$Mass: 0.2
$Velocity: 450.0 ;; speed of the weapon (initially) -- may or may not change
$Fire Wait: 0.3 ;; in seconds
$Damage: 8
$Armor Factor: 1.0
$Shield Factor: 1.0
$Subsystem Factor: 1.0
$Lifetime: 2.0 ;; How long this thing lives
$Energy Consumed: 0.30 ;; Energy used when fired
$Cargo Size: 0.0 ;; Amount of space taken up in weapon cargo
$Homing: NO
$LaunchSnd: 100 ;; The sound it makes when fired
$ImpactSnd: 85 ;; The sound it makes when it hits something
$Flags: ("in tech database" "player allowed")
$Icon: iconSD4
$Anim: SD4
$Impact Explosion: none
That'll give you one of them at least!
