With a dull game going nowhere Dorman replaced Alan Johnston on the hour-mark then missed two good chances within five minutes.
The Paisley midfielder twice had only Well keeper John Ruddy to beat but missed the target on both occasions leaving the home side without a win in their past 10 SPL matches, although the draw took them four ahead of bottom side Falkirk.
However, Buddies' boss Gus MacPherson must find a solution to their scoring problems if they are to draw away from the relegation battle.
This was a game between two clubs at opposite ends of the form spectrum.
However, there was little between the sides in an enterprising and open start to the game which saw both defences required to stand firm.
However, there was no early threat to either goal, though, and as the match settled down enterprise gave way to a dour, tactical battle.
The first real effort on goal took 18 minutes to arrive with Ruddy saving comfortably from Graeme Carey's long-range effort.
Fir Park midfielder Giles Coke tried his luck at the other end a minute later and after Saints goalkeeper Paul Gallacher spilled the low drive, Buddies' defender David Barron had to complete the clearance.
With the ball all too often in the air it was becoming difficult for either side to grab hold of the game.
On the half-hour mark, with still no sign of a breakthrough, Paisley striker Michael Higdon got on the end of Jack Ross's cross from the right but his header lacked the pace to worry Ruddy.
Moments later Coke had another effort from 30 yards which arrowed straight at Gallacher who gathered this time with ease.
The game showed slight signs of improvement as it nudged towards the interval. After St Mirren midfielder Hugh Murray's delivery from the right was flicked on by Craig Dargo, Higdon failed to connect at the back post and a half-chance was gone.
Three minutes from the break Carey came close when he latched on to a Murray pass at the edge of the box and fired just wide of the target before Paisley defender John Potter drew a good save from Ruddy with an injury-time header.
Jim O'Brien and Jamie Murphy replaced Ross Forbes and Coke at the interval as Well boss Craig Brown tried a change of approach.
However, it was Higdon who had the first effort on goal five minutes after the restart but again his header from Ross's looping cross was too tame to cause Ruddy an real concern.
The stalemate continued and only Carey, on loan to St Mirren from Celtic, was showing any real class.
On the hour-mark, Billy Mehmet and Dorman were introduced at the expense of Higdon and Johnston and seconds later Dorman, with his first touch of the ball, missed the target from 14 yards with only Ruddy to beat. In the 66th minute Dorman missed another great chance when a Dargo flick sent him clean through on goal.
The Buddies' midfielder bore down on Ruddy but slashed wildly at his shot and the ball flew wide.
Dorman had showed why he had only notched twice this season so far and those misses seemed to give Motherwell a renewed impetus.
The visitors brought on Michael McGlinchey for Chris Humphrey but it was Ruddy who was again called in to action, plucking Garry Brady's 25-yard volley out of the air.
Near the end, Dorman set up fellow substitute Stephen O'Donnell, on for Ross, but his header from 12 yards was cleared off the line by Steven Hammell and again, Saints were foiled.