Adam, The simple answer is yes, you can practise in a sitting position. Try to maintain the upper body alingments (for the spine, chest, head, etc.) with your feet flat on the ground–if you can–and the same width as your standing position. Rest your hands on your knees or thighs, keeping the armpits slightly open. Then, go through the sinking as you would in your standing practice. This can be done whenever you have any issues with your legs, stamina or illness, and allows you to continue practising.
When lying down, you would still sink your qi from the head, through your body to your feet, and from the soles of your feet drop down into the earth. This can be challenging for two reasons: 1) you don’t have the support of gravity since you’re horizontal instead of vertical; and 2) people generally fall asleep or space out more so than when they are sitting or standing. So sitting is better than lying down, but if your body is compromised and lying is all you can do, then it’s better than nothing.